A series of studies designed to identify dietary constituents affecting cataract incidence in rats with hereditary retinal dystrophy are being conducted. Both purified and natural ingredient diets varying in nutrient and ingredient composition are being evaluated. The age of onset and the incidence of cataracts can be influenced by dietary alterations. It appears there is a significant increase in the incidence of cataracts when fish meal is used in purified and natural ingredient diets. Although fish meal contains a considerable amount of calcium, results of studies where graded concentrations of calcium were fed in purified diets did not implicate this as the factor influencing the incidence of cataracts.